Surface Alteration of Titanate Ceramics in Aqueous Media
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379 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
McGlinn et al. [7]. After 56 days the samples were covered in a tens-of-microns thick layer of anatase [8]. Similar results were obtained on inactive samples of perovskite doped with Nd/Ce. However no such layers were evident in the work of McGlinn et al.[7]. The aims of the present work were to (a) extend the ERDA work [5,6] on Synroc-C reacted in D 20 by using smoother surfaces and higher temperatures, as well as to compare ERDA data with SIMS results; (b) use SIMS to see whether rare earths built up on Synroc surfaces exposed to water, as reported for sphene glass-ceramics [9]; (c) use alphaspectroscopy to check if actinides build up on Synroc surfaces after reacting with water; (d) study zirconolite-rich versions of Synroc designed for actinide immobilisation, and (e) explore the idea that the anomalous dissolution of the doped perovskite [8] derived from traces of fluoride in the solution. EXPERIMENTAL Synroc-C samples were made by the standard hot-pressing route [2] and the samples for dissolution studies were polished to a 0.25 pm diamond finish. Some samples were ground further with amorphous silica for several days and the surface roughness was reduced from - 25 nm to - 5nm, as studied by a Tencor Alpha-step 200 profilometer. Samples of zirconolite-rich Synroc [13], designed for PuO 2 immobilisation but containing CeO 2 substituted on a molar basis for PuO 2, were prepared by standard Synroc-C methods, with final consolidation effected by either graphite-die hot-pressing at 1250'C, or by sintering in air at 1300'C. The corresponding dissolution samples were polished to a 0.25 Pim diamond finish. The perovskite samples were made in polycrystalline form by similar means. X-ray diffraction measurements were performed with a Siemens D-5000 instrument. SEM was conducted with a JEOL JSM-6400 machine run at 15 kV and fitted with a Tracor Northern MICRO-ZII X-ray detector and a Series II TN5502 system to carry out energydispersive elemental analysis. D20 treatment on Synroc and Synroc phases was carried out for periods of up to 30 days in Teflon vessels at 150'C and in steel pressure vessels at the higher temperatures, up to 250'C. Measurements in deionised water as well as dilute HCI (pH = 2), with or without an admixture of 0.001 M F-, were made by modified MCC-1 methods at 90'C in Teflon containers. Solution analyses were carried out by ICP-MS. SIMS measurements were carried out for D20 and rare earths. For D 20 analysis, 10 keV Cs' primary ions were used, monitoring negative secondary ions of 160, 180 and mass 18.01. The primary ion beam was focussed to a spot of around 40 Pm in diameter, corresponding to a current of - 15 nA and raster scanned over an area of 250 x 250 Pim. Secondary ions from an area 8 pm in diameter within the centre of the sputtered area were admitted into the spectrometer for analysis. The instrument was operated with a mass resolution of - 2000 to separate "80 from mass 18.01. The depth profiles obtained for mass
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